1. Field of the Invention
The present invention relates to a laser scanning unit, and especially a laser scanning unit applied to a laser printer without a housing.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram of a laser printer 10 according to the prior art. The laser printer 10 includes a casing 12, an organic photo conductor 14, a charged roller 16, a laser scanning unit 18, a toner cartridge 20, a developer roller 22, a transfer roller 24, a fuser 26, and a discharge unit 28. The casing 12 is for shielding internal components of the laser printer 10. The organic photo conductor 14 is installed inside the casing 12 in a rotatable manner. The charged roller 16 installed inside the casing 12 distributes charges on the surface of the organic photo conductor 14. The laser scanning unit 18 installed inside the casing 12 projects laser beams onto the surface of the organic photo conductor 14. The toner cartridge 20 installed inside the casing 12 stores toner. The developer roller 22 installed inside the casing 12 and disposed at one side of the toner cartridge 20 is capable of absorbing toner from the toner cartridge 20. The transfer roller 24 installed inside the casing 12 connects to a negative bias (not shown). The fuser 26 installed inside the casing 12 melts a plastic portion of the toner. The discharge unit 28 installed inside the casing 12 discharges the charges on the organic photo conductor 14.
The printing procedures and principles of the laser printer 10 are introduced below. First, the charged roller 16 distributes electrons and positive charges on the surface of the organic photo conductor 14. Then, the laser scanning unit 18 projects laser beams onto a specific region of the surface of the organic photo conductor 14. Because the organic photo conductor 14 is composed of highly-photoconductive material, when the specific region of the surface of the organic photo conductor 14 receives the laser beams emitted from the laser scanning unit 18, the collisions between photons of the laser beams and electrons of the highly-photoconductive material ionize the electrons and make the specific region conductive, so as to form an electrostatic image with lower voltage. Voltage at a position of the electrostatic image is related to the frequency of the laser beam. In other words, the higher the frequency of the laser beam projected onto a point, the lower the voltage of the point, and vice versa. Subsequently, when the organic photo conductor 14 rotates to a position where the electrostatic image is adjacent to the developer roller 22, because the developer roller 22 has already absorbed charged toner stored in the toner cartridge 20, parts of the electrostatic image of certain voltages will absorb the charged toner from the developer roller 22, and the lower the voltage of a point on the electrostatic image is, the more charged toner the point will absorb. Next, when the organic photo conductor 14 rotates to a position where the electrostatic image is adjacent to the transfer roller 24, because the negative bias the transfer roller 24 is electrically connected to is much lower than the voltage at any position on the electrostatic image (meaning the absolute value of the minus bias is greater than the absolute value of the voltage at any position on the electrostatic image), the transfer roller 24 is able to transfer the charged toner on the electrostatic image to a print medium 11 between the transfer roller 24 and the organic photo conductor 14. At this time, the toner stays on the print medium 11 temporarily because of the attraction of static electricity, therefore any vibration of the laser printer 10 or other external forces will shift the toner on the print medium 11. But after the print medium 11 passes through the fuser 26, the heat generated by the fuser 26 will melt the plastic portion of the toner on the print medium 11 so as to make the toner stick to the print medium 11. After the transfer roller 24 transfers the toner on the electrostatic image to the print medium 11, and the organic photo conductor 14 rotates to a position where the electrostatic image is adjacent to the discharge unit 28, the discharge unit 28 will discharge the charges on the electrostatic image completely, so that when the organic photo conductor 14 rotates to a specific region (the electrostatic image has already disappeared) adjacent to the charged roller 16, the charged roller 16 is able to redistribute charges on the surface of the organic photo conductor 14. In addition, the laser scanning unit 18 comprises an optical system module 30, an optical scanning module 32, a curved lens 33, and a reflector module 34. The optical system module 30 generating laser beams according to print data comprises a laser beam source 36 for emitting the laser beams, and an optical optimizing element 38 for optimizing the laser beams emitted from the laser beam source 36. The optical scanning module 32 is for reflecting the laser beams generated by the optical system module 30. The curved lens 33 for reflecting the laser beams reflected from the optical scanning module 32 is an f-θ lens. The reflector module 34 is for reflecting the laser beams reflected from the curved lens 33 to the organic photo conductor 14.
Please refer to FIG. 2 and FIG. 3. FIG. 2 is a structure diagram of the laser scanning unit 18 according to the prior art, and FIG. 3 is a diagram of internal components of the laser scanning unit 18 according to the prior art. The conventional laser scanning unit 18 is designed as a module. The conventional laser scanning unit 18 includes a housing 40 for shielding internal components of the laser scanning unit 18. The optical system module 30, the optical scanning module 32, and the reflector module 34 are all installed on the housing 40, and the housing 40 is fastened on a frame 42. The frame 42 has a mechanism interface for connecting the laser printer 10, e.g. connecting to the casing 12 in order to fasten the whole laser scanning unit 18. However, in such a design in which the internal components of the laser scanning unit 18 are integrated inside the housing 40, and the housing 40 is installed on the frame 42, the housing 40 makes the assembly more complicated.